1. Field of the Invention
This invention concerns a magneto-optic recording/reproduction apparatus for recording information or reproducing recorded information on an optical disk, and in particular concerns a magneto-optic recording/reproduction apparatus wherein a stable focusing servo mechanism can be applied even if the wavelength of the optical source suddenly varies.
2. Description of the Prior Art
In a conventional magneto-optic disk apparatus which can perform recording and reproduction, the light beam from a semiconductor laser is made to converge by an objective lens on a magneto-optic disk so as to record or reproduce a signal.
However, the wavelength of the semiconductor laser shifts when the laser output power varies. As the laser output power used for recording is high and the output power used for reproduction is low, the wavelength shifts instantaneously by several nanometers when a change-over is made from reproduction to recording. If the optical system has a chromatic aberration, therefore, this wavelength shift causes a sudden displacement of the focusing point of the laser beam in the direction of the optic axis. Moreover, the sudden displacement cannot be corrected by a focusing servo mechanism in a quick enough manner.
FIGS. 14-19 show the relation between the displacement of the focusing point due to the wavelength shift and a focusing error signal. In the figures, (1) indicates reproduction, (2) indicates the situation immediately after a change-over is made from reproduction to recording, and (3) indicates recording.
First, the positional relationship between the objective lens and the disk, and the shape of the spot on a photodetector for detecting the focusing error, will be described with reference to FIGS. 14 and 15.
In the following description, the astigmatism method (cylindrical lens method) is used to detect the focusing error signal. In the astigmatism method, a light beam reflected by the disk is given astigmatism by a cylindrical lens, and is received by a photodetector divided into four regions. The focusing error signal is then obtained by summing the outputs from diagonally opposite regions of the photodetector, and taking the difference between the two sums.
During reproduction (long wavelength, low power) or recording (short wavelength, high power), as respectively shown in FIGS. 14(1), (3), the focusing point of the laser beam coincides with the recording surface of the magneto-optic disk MOD, and the spot on the photodetector is circular as shown in FIGS. 15(1), (3). In these situations, the outputs from the photodetector regions are equal and the focusing error signal is 0. Immediately after a change-over is made, however, the wavelength varies although the objective lens remains in the same position. The focusing point is therefore displaced by an amount d0 from the recording surface of the magneto-optic disk MOD as shown in FIG. 14(2), the spot on the photodetector becomes elliptical as shown in FIG. 15(2), and a positive or negative focusing error signal is output.
FIG. 16 shows the relation between the focusing error signal and the focusing point of the laser beam. The x axis represents the position of the focusing point of the laser beam, x=0 corresponding to the recording surface of the magneto-optic disk MOD. The y axis on the other hand represents the focusing error signal. The objective lens is adjusted such that the focusing error signal is 0, i.e. such that the focusing point of the laser beam coincides with the recording surface of the magneto-optic disk MOD.
During reproduction, as the power of the laser beam is low, the proportion by which the focusing error signal varies with respect to the displacement of the focusing point of the laser beam is small, so the relation between the two parameters is represented by the line A which has a small slope. On the other hand during recording, as the power of the laser beam is high, the aforesaid proportion is large and so the relation between the two parameters is represented by the line B which has a large slope.
When a change-over is made from reproduction to recording, the focusing point of the laser beam which was at the position shown in (1), is instantaneously displaced to the position shown in (2) due to the variation of wavelength, and then returns to the position shown in (3) due to the movement of the objective lens. The focusing error signal generated by the displacement d0 of the focusing point at the instant of change-over is E0.
FIG. 17 shows the relation between the position of the objective lens (horizontal axis) and the amplitude of the magneto-optic disk signal AP. The amplitude AP of the magneto-optic recording signal is a maximum (level 1 in the figure) when the focusing point of the laser beam coincides with the magneto-optic disk, and it declines along the curves A', B' as the focusing point moves away from the disk. Herein, the curve A' corresponds to the case of reproduction, and the curve B' to the case of recording.
The inflection points on the curves show the positions of the objective lens when the focusing point of the laser beam coincides with the recording surface of the disk. To make the focusing point coincide with the recording surface, the objective lens must be displaced by a distance d0 when a change-over is made from reproduction to recording. When the objective lens is at a point (1) on the curve A' such that the focusing point coincides with the recording surface using the reproduction wavelength, and a change-over is made to the recording wavelength, the amplitude of the magneto-optic recording signal falls to a point (2) on the curve B'. The objective lens is then driven by the focusing servo mechanism such that the focusing point coincides with the recording surface at the point (3) on the curve B' using the recording wavelength, and the amplitude of the magneto-optic recording signal is again a maximum.
FIG. 18 shows the variation of the focusing error signal FE with time, and FIG. 19 shows the variation of amplitude of the magneto-optic recording signal with time. The focusing error signal is E0 due to a change-over from reproduction to recording, and the magneto-optic recording signal falls by an amount corresponding to E0. A change-over time t0 is required for the focusing error signal to return to 0, and for the magneto-optic recording signal to return to its maximum value.
This conventional magneto-optic recording/reproduction apparatus is adjusted so that the focusing error signal is 0 when the focusing point coincides with the optical disk. A large focusing error signal is therefore generated due to the displacement of the focusing point when there is a change-over of the laser wavelength.
A servo mechanism adjusts the objective lens so that the focusing error signal returns to 0. A considerable time is however required until the focusing point coincides with the disk, and during this time the level of the disk signal may fall so that accurate recording and reproduction are not possible.